Substitute for dry sherry called for in French onion soup?

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If you're trying to go alcohol-free you can also substitute white wine vinegar. It adds a bit more acidity than the wine would. If you prefer a sweeter flavor then white cranberry or white grape juice works as well.

My suggestion of a substitute would depend upon why you asked for one. Are you trying to avoid alcohol? or just want to know if you can sub another bevvie you might have on hand. Also, are you making the soup with beef or chicken stock?

To maximize the flavor of the onions, I would stick with dry white vermouth.

Ditto the suggestions above; or, if you'd like a flavor more related to sherry, madeira works well (small world, I have some in the fridge right now, made that way. Pretty tasty, if I do say so myself ;-)

Aside from white or red wine, dry vermouth, cognac or brandy would work.

You could just leave it out or use white wine. It won't taste the same, of course, but it will still be delicious.